The present invention relates to a system for detecting the position of an objective lens, and more specifically to a system in which light rays, emitted from a light emitting element, are directed toward a mirror fixed to the objective lens, and light sensing elements sense the reflected rays from the mirror, thereby detecting the position of the lens.
Information recording/retrieving apparatuses have recently been developed for improvement. These apparatuses are designed so that information is recorded on or retrieved from an optical disk, optical card, or other information recording medium (hereinafter referred to simply as optical disk), by means of a light beam. In these apparatuses, the light beam must be focused accurately on the optical disk so that tracking guides, formed on the optical disk, can be traced accurately by the beam. To attain this, the conventional information recording/retrieving apparatuses is provided with a focusing servo unit for detecting a focusing state and keeping an objective lens always in the focusing state, and a tracking servo unit for detecting a tracking guide and directing the lens toward the guide at all times. Conventionally, the focusing state has been detected by the astigmatic method using an astigmatic lens, the critical-angle method using a total-reflection prism, or the knife-edge method utilizing a knife edge. In these methods, the astigmatic lens, prism, or knife edge is disposed in the optical path of the light beam, reflected from the optical disk, and the beam, transmitted through the lens or the like, is delivered to a photodetector for focus detection. According to a method of track detection, moreover, a diffraction image of the tracking guide is detected. Also in this method, the light beam reflected by the optical disk is led to a detector for detecting the diffraction image.
Thus, according to these focus and track detection methods, the focus and track are detected by means of the light beam, reflected from the optical disk. Therefore, if the optical disk has any scratches or dust thereon, or if it vibrates from any causes, error signals may possibly be generated from a photodetector for focus and track detection, thus moving the objective lens in an unexpected direction, and sometimes even causing it to run against the optical disk. In response to the error signals, moreover, the objective lens may be directed toward an undesired tracking guide, or it may take too much time to redirect the lens toward a desired tracking guide. In some cases, oscillation may be caused in a servo loop, thus inducing the objective lens to vibrate.